The invention more specifically relates to an improvement to the steam generator described in FR-A-2 477 265. The latter describes a steam generator in which the secondary water, generally called feed water, is injected by vertical tubes into the upper, open and flared end of a semicircular feed space. The latter is formed between an inner envelope surrounding the bundle of U-shaped tubes of the generator and a skirt externally duplicating part of said envelope surrounding the branches of the tubes in which the primary water redescends towards the discharge collector for said primary water. Mixed said feed water circulating from top to bottom in the feed space is part of the water, called recirculation water, flowing out of water-steam separators positioned in the upper part of the internal envelope of the exchanger. The rest of the recirculation water flows downwards between the outer envelope of the steam generator and the assembly formed by the inner envelope and the skirt.
This configuration makes it possible to ensure homogeneous preheating of the feed water by the recirculation water within the feed space, prior to the water rising into the inner envelope along the tubes of the group in order to vaporize on contact therewith. For this reason, the feed space is also called an economizer.
In such a steam generator, the supply or feed tubes by which the feed water arrives are generally in the form of an inverted J. Moreover, the skirt approximately follows the contour of the outer envelope of the generator, in such a way that its upper part is flared in order to give the feed space at this level the shape of a funnel. Due to these special shapes of the feed tubes and the feed space and also due to the high speed of the feed water jets passing out of said tubes, eddies can occur in the upper part of the feed space.
These eddies can rise above the feed torus to which are connected the feed tubes and can entrain a large quantity of a mixture formed from saturated recirculation water and feed water. Part of this mixture, at a temperature below the saturation point, then redescends through the space formed between the outer envelope of the generator and the assembly formed by the inner envelope and the skirt. The other part of the mixture, at a temperature above that of the feed water, is injected into the feed space.
The consequence of this phenomenon is to reduce the efficiency of the preheating of the steam generator, which can reduce the pressure therein. The exchange surface of the steam generator is consequently not used in an optimum manner, and consequently its thermal efficiency is reduced.